Saddleback Journal of Biology - Saddleback College

Spring 2010 Biology 3B Paper
between the birds, as the years of intense daily
competition for unlimited supplies might have, at the
risk of anthropomorphizing hummingbird learning
abilities, “taught” them or “shown” them that no net
benefits resulted from territorializing unlimited
resources. For instance, territoriality displays were
still quite prevalent throughout the eight hours of
footage, however only high-and-low-intensity
chasing was, as far as researchers could determine,
the most effective method for defenders to protect
their territory from invaders. Gorget displays were
the overwhelmingly most common behavior
exhibited, yet only 12 instances of such displays were
effective enough to drive away an intruder.
Otherwise most hummingbirds simply ignored each
others’ warning and fed regardless. Potential
defenders more often did not pursue invaders and
simply began to feed without further aggression.
With such competitive density, it could be argued
that the invading hummingbirds were not intimidated
by simple low-cost displays, yet defending
hummingbirds did not pursue more energetically
demanding displays because the benefits to
exclusivity did not outweigh the costs associated with
such rigorous and constant defense.
Other studies (Ewald and Carpenter, 1978 and Ewald
and Orian, 1983) contained numerous feeders and an
abundant food supply, yet the studied hummingbirds
still exhibited frequent high-intensity territorial
displays. Carpenter (1987) discussed the possibility
that the territorial defenses were still utilized by those
hummingbirds because though food productivity was
abundant in the area of study, regionally there was a
food productivity limitation driving the
hummingbirds to defend high-resource food sources.
(It is important to note that researchers could find no
further definition of what constituted a “region” by
which to understand where the boundaries of regional
food productivity lie. Thus, researchers interpreted
the definition as the hummingbird population’s home
range, for ease of comparison). However, in the area
of study for this experiment, the 3.5 acre grove is
situated in an agricultural setting with an approximate
three square mile radius of nearly uninterrupted
flowering citrus and avocado trees. If the
hummingbirds seek nourishment outside of the
ACFA, the same abundant food productivity
environment exists year-round well beyond their
home range.
As with any behavioral study, further data
could help to resolve or explain inconsistencies
between results. A total of two days of resource
depression might not have been a sufficient length of
time to capture behavioral differences considering the
complex social dynamics and energy requirements of
hummingbirds. In addition, it is possible that
researchers did not sufficiently lower the availability
of food sources in accordance to Carpenter and
MacMillan (1976) lower threshold. However,
because energy budgets were not the main focus of
this study, a per calorie comparison was not
calculated to determine what the lower threshold
might be for this population. Further research on the
effects of ACFA resource depression and
territoriality should include more hours of
observation, a threshold calculation, and a lowresource
acclimation period of at least one week prior
to data collection in order to ease the transition from
abundance to resource depression.
Literature Cited
Brown, Jerram L., 1969. Territorial Behavior and
Population Regulation in Birds. The Wilson Bulletin.
81(3): 293-329.
Brown, Jerram L., Orians, Gordon H., 1970. Spacing
Patterns in Mobile Animals. Annual
Review of Ecology and Systematics. 1: 239-262.
Carpenter F. Lynn, MacMillen R. E., 1976.
Threshold Model of Feeding Territoriality and Test
with a Hawaiian Honeycreeper. Science. 194: 639-
642.
Carpenter, F. Lynn, 1987. Food Abundance and
Territoriality: To Defend or Not to Defend?,
American Zoologist. 27 (2): 387-399.
Carpenter, F. Lynn, Hixon, A., Paton, D. C., 1989.
Regulating Body Mass Changes to Fitness in
Hummingbirds. Bulletin of the Ecological Society of
America. 70: 77
Eberhard, Jessica R., Ewald, Paul W., 1994. Food
Availability, Intrusion Pressure and Territory Size:
An Experimental Study of Anna’s Hummingbirds
(Calypte anna). Behavioral Ecology and
Sociobiology. 34 (1): 11-18.
Ewald, Paul W., Carpenter, F. Lynn, 1978. Territorial
Responses to Energy Manipulations in the Anna
Hummingbird. Oecologia. 31(3): 277-292.
Ewald, Paul W., Orians, Gordon H., 1983. Effects of
Resource Depression on Use of
Inexpensive and Escalated Aggressive Behavior:
Experimental Tests Using Anna
Hummingbirds. Behavioral Ecology and
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Kodric-Brown, A., and Brown, J.H., 1978. Influence
of Economics, Interspecific Competition, and Sexual
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Saddleback Journal of Biology
Spring 2010